1. Field of the Invention
The present invention relates to a time division MULTIPLEXED passive optical network, and more particularly, to a method and system for fairly allocating a differentiated bandwidth based on shared multicast traffic in the transmission of downstream traffic from an optical line terminal (OLT) to an optical network unit (ONU).
2. Description of the Related Art
Due to ongoing growth of video-based applications requiring real-time data transmission and provision of integrated communication and broadcasting service, the total amount of Internet traffic has been rapidly increasing. In order to efficiently cope with this traffic increase, network providers apply Wavelength Division Multiplexing (WDM) technology to inter-city backbone networks and metro networks, thereby continuously increasing their transmission bandwidths.
In contrast, subscriber networks, in which the traffic transmitted from the backbone networks is distributed to end subscribers, have used conventional transmission technology based on Very-high-data-rate Digital Subscriber Line (VDSL) and cable modem together with high-speed Ethernet based technology. These subscriber network technologies can be deployed only in short distance area, and narrow transmission bandwidth is a basic bottleneck to provide the integrated services that are intensively discussed at present. In order to solve this problem, optical subscriber network technologies are being actively developed. The optical subscriber network technologies are directed to efficiently providing end subscribers with the transmission bandwidths required for the integrated services.
These optical subscriber network technologies include Time Division Multiplexed-Passive Optical Network (TDM-PON) technology and Wavelength Division Multiplexed-Passive Optical Network (WDM-PON) technology. The TDM-PON has a point-to-multipoint structure in which an optical line terminal (OLT) is connected with a plurality of optical network units (ONUs) through a passive optical splitter (POS), and a single transmission wavelength is shared with the plurality of ONUs in an optical layer. In the TDM-PON, downstream data transmission is carried out by TDM, while upstream data transmission is carried out by Time Division Multiple Access (TDMA) based on bandwidth reservation.
Unlike the TDM-PON, the WDM-PON has a logical point-to-point structure in which an individual transmission wavelength is allocated to each ONU. Data transmission between the OLT and the ONUs is independently performed without a time division process. Accordingly, the WDM-PON can provide subscribers with wider transmission bandwidths.
However, the deployment cost of WDM-PON is still high due to an expensive transmission system and optical devices, and thus it is predicted to take some time before it can be put to practical use. In contrast, the TDM-PON can make efficient use of the same wavelength through time division, and furthermore requires relatively inexpensive equipment. Thus, the TDM-PON has drawn attention as next-generation optical subscriber network technology.
According to a framing and multiplexing method of the data link layer, TDM-PON technology can be divided into E-PON, G-PON, and B-PON technology, all of which have the same basic up/downstream transmission control concept. First, in the case of upstream transmission, because the data transmission from a plurality of ONUs and Optical Network Terminals (ONTs) to an OLT, a common destination, is performed through a shared link, a proper Multiple Access Control (MAC) technique is required in order to prevent data collision. To this end, the ONUs and ONTs generally reserve the bandwidth of transmission window required for the next transmission period on the basis of the total amount of data stored in buffers, and the OLT arbitrates this reservation and then allocates a transmission timeslot, i.e., an upstream bandwidth. Hence, it is possible to maintain high network efficiency, and fairly allocate bandwidth to the ONUs as well. At this time, due to point-to-point communication in which the primary destination of a data frame transmitted from each ONU for a timeslot period of time is the OLT, which means no multicast traffic exist in TDM-PON area, it is possible to easily maintain fairness of the bandwidth allocation by controlling the timeslot size.
Unlike upstream transmission, downstream transmission is performed as follows. All data frames transmitted from the OLT are split in the optical layer through the POS and then broadcasted to all of the ONUs and ONTs. Each ONU filters the broadcast frames to receive only necessary frames on the basis of a destination address in a MAC layer. At this time, if all traffic is unicast traffic, i.e., frames directed to only a single destination, similar to upstream transmission, the OLT can allocate downstream transmission bandwidth to ONUs fairly by controlling timeslot size of ONU.
However, in practice, in the case of downstream transmission in the TDM-PON, there is always a large amount of multicast traffic due to multicast Video On Demand (VOD) and broadcasting video service, and this multicast traffic is simultaneously shared with the numerous ONUs through OPS. As such, in the case of downstream transmission, when the transmission timeslots are merely allocated in a uniform way, it is impossible to guarantee fair allocation of bandwidth.